The present invention relates to monitoring and measuring gases, such as constituent gases in flue gases in combustion systems, including systems having sensors used in combustion applications, such as boiler, furnace, combustion gas turbine or fossil combustor applications.
To understand the nature and operation of the invention it may be helpful to consider some exemplary applications. For instance, in numerous industrial facilities or environments a hydrocarbon fuel is burned in a combustor (e.g., a boiler or furnace) to produce heat to raise the temperature of a fluid. For the combustor to operate efficiently and to produce an acceptably complete combustion having byproducts/products of combustion that fall within the limits imposed by environmental regulations and design constraints, all of the individual burners should be operating cleanly and efficiently, and all post-flame combustion control systems should be properly balanced and adjusted.
Emissions of unburned carbon, nitric oxides (NO, NO2, NOx), carbon monoxide or other byproducts commonly are monitored to ensure compliance with environmental regulations. As used herein and in the claims, the term nitric oxides shall include nitric oxide (NO), nitrogen dioxide (NO2), and nitrogen oxide (NOx, where NOx is the sum of NO and NO2). The monitoring of emissions heretofore has been done, by necessity, on the aggregate emissions from the combustor (i.e., the entire burner array—taken as a whole). Some emissions, such as the concentration of gaseous combustibles in hot flue gases, are difficult and/or expensive to monitor on-line and continuously. These emissions are typically measured on a periodic or occasional basis. When a particular combustion byproduct is found to be produced at unacceptably high concentrations, the combustor should be adjusted to restore proper operations. However, measurement of aggregate emissions, or measurement of emissions on a periodic or occasional basis, provides little, if any, useful information regarding what particular combustor parameters should be changed to effect such an adjustment.
Three main combustion variables, namely O2, CO and NOx should be continuously monitored to optimize the combustion process and to achieve a goal of providing maximum efficiency at the minimum level of emissions. Solid electrolyte (e.g., zirconia) based combustion sensors are well known and commonly used in fossil combustors to measure oxygen and combustibles (commercial suppliers include Rosemount Analytical, Ametek Thermox, and Yokogawa). These sensors are usually used with reference air applied to one of two electrodes. In most cases the existing sensors are extractive and require high maintenance.
Recently some suppliers have introduced oxygen sensors that do not utilize a continuous supply of reference gas. Instead, these sensors have a sealed internal electrode filled with a mixture of metal/metal oxide that generates a constant partial pressure of O2 inside the sealed volume.
There are a number of methods to measure flue gas combustibles (primarily CO) using solid electrolytes. One of the methods is based on using a fluctuating signal in an in-situ potentiometric solid electrolyte cell directly positioned in the high temperature flue gas stream, as described in U.S. Pat. No. 6,277,268 (Khesin et al.), entitled “System And Method For Monitoring Gaseous Combustibles In Fossil Combustors” (referred to herein as the '268 patent). These sensors are relatively simple in design and provide an immediate response. An example of an existing sensor in production and available on the market is the MK CO sensor manufactured by the General Electric Reuter-Stokes Company of Twinsburg, Ohio.
The '268 patent discloses, among other things, an apparatus for monitoring changes in a concentration of gas molecules of at least a first type in an environment. The apparatus includes a mass of material and first and second electrodes. The mass of material is permeable to ions formed when gas molecules of the first type are ionized. The first and second electrodes are arranged on the mass of material such that, when a concentration of the gas molecules of the first type at the first electrode is different than a concentration of the gas molecules of the first type at the second electrode, gas molecules of the first type are ionized at the first electrode to form ions which flow from the first electrode to the second electrode via the mass of material and are recombined at the second electrode to form gas molecules of the first type, thereby generating a signal between the first and second electrodes. Each of the first and second electrodes is in fluid communication with the environment.
More specifically, the '268 patent discloses a system for monitoring changes in a concentration of oxygen present in an environment, the system includes at least one Nernstian-type gas sensor. The sensor includes a mass of solid-electrolyte material and first and second electrodes. The first and second electrodes are disposed on the mass of solid-electrolyte material to generate a signal therebetween indicative of a difference between an oxygen concentration at the first electrode and an oxygen concentration at the second electrode. Each of the first and second electrodes is in fluid communication with the environment. However, the Nernstian-type gas sensor may be used to monitor the concentration of any number of gases. The mass of material included in the sensor is permeable to ions formed when gas molecules of a first type are ionized. A signal is generated between the at least first and second electrodes in response to changes in the concentration of the gas molecules of the first type in the environment. Further, the sensor may be free of a temperature control device.
The '268 patent further discloses a method for calibrating a gas sensor, the method including supplying each of a first gas having a first profile and a second gas having a second profile, which is different that the first profile, to the gas sensor in a predetermined sequence. The gas sensor or a signal analyzer associated therewith is adjusted based upon an output signal of the gas sensor to calibrate the gas sensor. An apparatus for calibrating a gas sensor includes a switching system and a sequencer. The switching system is in fluid communication with each of a first tank having a first predetermined gas profile and a second tank having a second predetermined gas profile. The sequencer causes the switching system to supply gas from each of the first and second tanks to the gas sensor in a predetermined sequence.
The '268 patent further discloses, such as at FIGS. 8A and 8B, a multi-piece pipe and connector arrangement for assembly and installation of the sensor in a field combustor application.
There are a number of methods to measure NOx in flue gas using Nernstian solid electrolyte sensors in the mixed potential potentiometric mode. In such designs, the analyzed gas, prior to reaching the measuring electrode, passes through a porous filter that enhances its sensitivity to NO or NOx, the sum of NO and NO2. The practical use of such “filtered” NOx sensors is difficult due to the effects of other components, primarily CO and O2.